Bag filters are generally used as filters for collecting the dust discharged from municipal refuse incinerators, coal boilers, metal melting furnaces, etc. In these furnaces, since the temperature of the exhaust gas reaches as high as 130 to 250° C., the filter media are required to be resistant against heat. Furthermore, depending on applications, the exhaust gases may contain chemicals, and in this case, the filter media are required to be resistant also against chemicals.
The conventional filter cloths used as filter media required to have such heat resistance and chemicals resistance are felts obtained by laminating ground fabrics and webs prepared using such materials as polyphenylene sulfide (hereinafter abbreviated as PPS) fibers, meta-aramid fibers, polyimide fibers, fluorine fibers or glass fibers, and treating the laminates by means of a needle punch, water jet or the like for entangling the fibers.
In recent years, for the purposes of improving the filtration efficiency and reducing the size of dust collectors, filter media having larger filtration areas are demanded, and it is considered to pleat the filter media for increasing filtration areas. However, the conventional felts have such problems that pleating is difficult since the conventional felts are insufficient in stiffness and that even if they are pleated, the pleated forms cannot be maintained.
As a method for solving the problems, U.S. Pat. No. 6,103,643 proposed a filter medium with higher stiffness obtained by impregnating a nonwoven fabric consisting of PPS fibers with a synthetic resin. However, the synthetic resin-impregnated nonwoven fabric of U.S. Pat. No. 6,103,643 has such problems that the stiffness declines at high temperature to lower the shape maintenance of pleats and that the chemicals resistance is insufficient. There is also a problem that if the fabric is exposed to high temperature for a long time, the strength declines remarkably.